This invention relates to a signal defect compensator which may be used in a system for reproducing recorded television image information.
In the reproduction of television video signals from information recorded, for example, on magnetic tape, magnetic discs or other types of recording media, a decrease or loss of the recorded signal information may be encountered, which is apparent to the viewer as a degraded picture. A principle form of such a defect is known as a dropout which may occur due to imperfections in the recording media or an accumulation of dirt or other debris at the junction where the playback/recording heads and the media meet. When such a dropout occurs, the signal recovered from the media generally suffers a sharp reduction in amplitude which appears in the picture seen by the viewer as random black and white streaks or flashes on the screen of the television receiver or other means utilized as a reproducer. In severe cases, such a dropout may occur for one or more entire lines of the television picture.
Since it is known that television image information is to some extent redundant from line to line, dropout compensators (DOC) have generally operated on the principle that since the image information is redundant from line to line, it is possible to compensate for a dropout by substituting information from a previous image line during the duration of the dropout. Known defect compensators, therefore, store image information from a previous line and insert the stored previous line information into the output video signal when a signal dropout occurs.
The loss of signal, generally characterized as a dropout, is typically detected by a signal amplitude detector circuit which produces a pulse with a duration corresponding to the loss of signal. The dropout pulse after suitable processing is utilized to operate a switching circuit which inserts the previously stored line information for the duration of the signal dropout. Signal dropout detectors of the form described above exhibit difficulty in accurately defining the actual signal dropout due to the nature of the video signal recovery from the recorded medium. Video playback systems generally contain de-emphasis circuits in the form of resistor-capacitor networks which are designed to compensate for the signal response of the recording medium. When a signal amplitude loss occurs, the de-emphasis circuits tend to integrate the signal amplitude changes so that short duration signal dropouts, which contain relatively little energy or lengthened only slightly, while long duration signal dropouts contain sufficient energy to fully charge the capacitors of the de-emphasis circuit to the peak amplitude of the signal disturbance, thereby causing the signal disturbance seen by the dropout detector circuit to persist beyond the actual signal dropout duration. Present defect compensation systems attempt to overcome this problem by stretching the dropout pulse by a comparatively long fixed duration that is sufficient to accommodate the expected worst case signal amplitude change. However, the amount of fixed pulse stretching required for a worst case condition is now excessive for short duration dropouts and causes a severely degraded picture quality where a series of short duration dropouts occur in close succession. The problem becomes even more severe where the short duration dropouts occur in time near the critical elements of the television waveform, such as sync and burst, where the extended dropout pulse may cause loss of these critical timing waveform elements. The present invention overcomes this problem by providing a proportional duration pulse stretching of the dropout pulse to compensate for the effects of the video signal recovery circuits.